The present invention pertains to fiber optical current and voltage sensors and, more particularly, to adaptive filters for use in such sensors.
One important aspect of electrical power systems deployed by the power generation industry is the ability to measure power carried over high voltage transmission lines. Power measurement has typically been performed on the high voltage side of the systems before the voltage is stepped down for distribution. However, the need is growing for more frequent and more accurate power measurements (e.g., voltage and current measurements) throughout the power distribution systems. Some recent innovations associated with making such power measurements involve the use of fiber optic sensors.
For example, fiber optic current sensors operate based on the Faraday effect. Current flowing in a wire induces a magnetic field which, through the Faraday effect, rotates the plane of polarization of the light traveling in the optical fiber wound around the current carrying wire. Faraday's law can be stated as:I=H dL  (1)where I is the electrical current, H is the magnetic field and the integral is taken over a closed path around the current. If the sensing fiber is wound around the current carrying wire with an integral number of turns, and each point in the sensing fiber has a constant sensitivity to the magnetic field, then the rotation of the plane of polarization of the light in the fiber depends on the current being carried in the wire and is insensitive to all externally generated magnetic fields such as those caused by currents carried in nearby wires. The angle, ΔΦ, through which the plane of polarization of light rotates in the presence of a magnetic field is given by:ΔΦ−V∫H dL  (2)where V is the Verdet constant of the fiber glass.
The sensing optical fiber performs the line integral of the magnetic field along its path, which is proportional to the current in the wire, when that path closes on itself. Thus, ΔΦ=VNI where N is the number of turns of sensing fiber wound around the current carrying wire. The rotation of the state of polarization of the light due to the presence of an electrical current can be measured by injecting light with a well-defined linear polarization state into the sensing region, and then analyzing the polarization state of the light after it exits the sensing region. Alternatively, ΔΦ represents the excess phase shift encountered by a circularly polarized light wave propagating in the sensing fiber.
This technology is related to the in-line optical fiber current sensor as disclosed in U.S. Pat. No. 5,644,397 issued Jul. 1, 1997, to inventor James N. Blake and entitled “Fiber Optic Interferometric Circuit and Magnetic Field Sensor”, which is incorporated herein by reference. Optical fiber sensors are also disclosed in U.S. Pat. No. 5,696,858 issued Dec. 9, 1997, to inventor James N. Blake and entitled, “Fiber Optics Apparatus and Method for Accurate Current Sensing” and U.S. Patent No. 6,188,811 to James N. Blake and entitled “Fiber Optic Current Sensor”, the disclosures of which are incorporated herein by reference.
These types of fiber optic current sensors have, for example, the advantage that they operate over a very wide dynamic range. This gives a single optical current sensor the potential to operate as both a protection device as well as a metering device. Applications in some power lines require the capability to measure currents as high as 170 kA and to handle transient currents with accuracies to within a few percent, while at the same time being able to measure a 1 A current level to within a few tenths of a percent accuracy. Other applications require wide dynamic range measurements of rms current. Still other applications require accurate measurements of a wide range of current harmonics in addition to the fundamental power frequency current for power quality determination.
In all of these exemplary applications, the fiber optic current sensor relies on its inherent linear and wide bandwidth operation to faithfully capture the true current values. However, the fiber optic current sensor also adds white noise to the sensed current. This noise can be electronic noise, but also contains a component of shot noise associated with the optical sensing mechanism. The presence of noise limits the functionality of the sensor to meet the aforementioned applications, especially when the true currents are at the low end of the dynamic range. In some cases, more sophisticated signal processing in the receiving instruments could allow for the white noise of the fiber sensor to be filtered out, and thus be of essentially no consequence for the application. However, because the receiving instruments were not designed with fiber sensors in mind, they do not always filter out the white noise, and thus yield erroneous measurements. For example, if the noise is of significant value compared to the true current, rms current calculations can be in gross error if the noise is not filtered out.
One way to address this noise issue is to carefully configure the number of fiber sensing turns, and the sensor bandwidth in the fiber optic current sensor to optimize the signal, noise, and linearity to the application. However, this approach leaves such a fiber optic current sensor potentially unable to achieve a full range of application, and also is not optimum from a cost perspective, as it requires the use of many turns of costly sensing fiber when the currents to be sensed are small. In addition, the number of sensing turns cannot easily be changed once the sensor is put into operation, so it is difficult to re-optimize the sensor once it has been installed.
Accordingly, it would be desirable to provide optical current (and voltage) sensors and sensing methods which address the foregoing issues.